1. Field of the Invention
The present invention is related to field electromagnetic fluid pumps and, more particularly, to an on-demand electromagnetic pump having a solenoid coil, position sensing coils and solid state circuitry for reciprocating a piston member.
2. Prior Art
The early reciprocating electromagnetic fluid pumps used mechanical or magnetically actuated switches to detect the end of the piston member's stroke. Activation of the switch applies power to the solenoid coil retracting the piston member back to its start position. Typical examples of such electromagnetic pumps are disclosed by L. C. Parker in U.S. Pat. No. 2,994,792 (August 1961) and E. D. Long in U.S. Pat. No. 3,361,069 (January, 1968). These switches are subject to mechanical failure as well as contact erosion and, therefore, were the primary causes of pump failure. An alternate approach to using mechanical switches was the use of an electronic oscillator which energized the solenoid coil at a predetermined interval. Some of the pumps used free running oscillators such as taught by Kofinh in U.S. Pat. No. 3,211,798 (December, 1965) or used a blocking oscillator as taught by Wertheimer in U.S. Pat. No. 3,381,616 (May, 1968). Pumps with electronic oscillators operated at a constant speed worked well for a relatively narrow band of output loads. However, the frequency of the oscillators had to be adjustable to compensate for changes in load conditions and were not well suited for applications where the load fluctuated randomly over a fairly wide range. One attempt to overcome this problem is taught by Woodward in U.S. Pat. No. 3,118,383 (January, 1964). In this patent, Woodward discloses the use of sensing coils to generate signals at one or both ends of the piston members stroke to prematurely initiate or terminate the output pulse of the multivibrator. Using this arrangement, Woodward is able to expand the operating load range of the electromagnetic fluid pump to higher loads, but the lower operating range was still determined by the natural frequency of the multivibrator. The electronic control disclosed herein overcomes the problems associated with mechanical switches and oscillating circuits discussed above.